Recent years have witnessed a mounting interest in the RIM technology, that is a technology by which an automative bumper, for instance, is manufactured by simultaneous polyurethane-forming reaction and molding of the resulting resin in a mold, and several versions of such technology have been disclosed in, inter alia, U.S. Pat. Nos. 4,254,069, 3,838,076, 4,216,543, 4,246,363 and 4, 269,945.
However, the technology of reacting a polyalkylenepolyol with a polyisocyanate in a mold cavity to give a polyurethane molding in situ not only encountered various difficulties due to the relatively poor heat resistance and coatability of the polyurethane but entailed a high production cost. For this reason, attention was paid to polyurea resins which are more heat-resistant and there has been proposed a technology for producing polyurea moldings which are superior to polyurethane moldings in mechanical strength, heat resistance and other physical properties. This technique comprises reacting an amine-terminated polyoxyalkylenepolyamine having an average molecular weight of at least 1500 with an amine terminal chain extender and a polyisocyanate in a closed mold. A method in this category is disclosed in Japanese Patent Application Laid-Open No. 1818643/83.
However, since a polyoxyalkylenepolyamine is more reactive to an isocyanate component than a polyether polyol, the cure reaction proceeds so fast that, unless mold charging is completed within, say, one second by the use of a machine dedicated solely to the polyurea RIM, no satisfactory molding can be obtained.
The reactivity of polyoxyalkylenepolyamine to a polyisocyanate may be decreased by converting its terminal amino groups from an aliphatic to aromatic nature. Thus, Japanese Patent Application Laid-open No. 273016/91 proposed to use an aromatic polyoxyalkylenepolyamine produced by the reaction of a polyoxyalkylenediepoxide with an aromatic amine. Alternatively, the reactivity of a polyoxyalkylenepolyamine may be decreased by converting its terminal amino groups from a primary to secondary amino group as taught in Japanese Patent Application Laid-open No. 311116/89 and No. 41310/90. U.S. Pat. No. 5,192,814 assigned to the assignee of this application discloses a polyurea RIM composition comprising a secondary polyamine produced by the Michael reaction of a primary polyamine with an unsaturated monomer. However, the known methods are not fully satisfactory in terms of initial stength characteristics of the resulting molding. Because of this, the molding often tends to be deformed or a piece thereof is torn off and remains within the interstices of the mold when the molding is ejected.
Since the length of time required for each molding cycle should be as short as possible for the polyurethane/polyurea RIM, an easy mold release operation is an important factor for performing a high productivity in the RIM. Usually this is accomplished by applying a mold release onto the inner surfaces of mold cavity. It is also known to incorporate an internal mold release into the resin material of RIM. By so doing, the application of an external mold release may be dispensed with or rendered persistent with a number of repeated molding cycles. See, Japanese Patent Application Laid-open No. 500418/85, No. 83215/86 and No. 88845/81. Typically, the internal mold release used therein is a higher fatty acid, a metal salt or amide thereof. However, the known internal mold release has certain defects in that it is not so effective in the polyurea RIM as in the polyurethane RIM and more importantly that the coatability of the resulting polyurea moldings is greatly impaired. Consequently, the molding requires a pre-treating step such as rinsing with trichloroethane before subjecting to a coating process in order to retain a good adherance to the coating film.
A need exists, therefore, for a polyurea resin composition which may ameliorate or eliminate the above defects.